In conventional conferencing systems there may be coupling between the speakers used to generate acoustic waves from far-end participants and microphones used to sense acoustic waves from near-end participants in a room. This coupling can create an undesirable effect often referred to as echo. Echo may be produced in a conferencing system where a distant participant is involved. The audio from a distant participant is produced at a speaker, which may be picked up at a microphone, and delivered again to the distant participant, who hears an echo of himself with a delay equal to two times the propagation delay over the carrier medium between the near-end and the far-end. Echo is not generally objectionable where the distant participant is located relatively close by (e.g., in the same building) but may become distracting where the echo delay is more than a few milliseconds.
To deal with echo, a traditional method is to use half-duplex operation. With half-duplex, the conferencing system detects the presence of far-side audio (from the distant participant) and effectively turn off the microphone to avoid the distant party's audio being echoed back. This method has the undesirable effect of preventing both parties from speaking simultaneously, and because the local participants cannot interrupt the far-side participant, a conversation tends to have a perceived unnatural flow.
Certain of the more advanced conferencing systems implement an echo canceler, which can effectively solve the problems of echo even where the conference proceeds in full-duplex. This echo cancelation is generally performed by predicting the signal received at the microphone based on the signal produced at the speaker. That predicted signal can be subtracted from the actual signal received at the microphone, theoretically leaving only the audio of any local participants as heard by the distant participants. Predicting the signal to subtract is complex, and is affected by the frequency responses of the speaker and the microphone, as well as echo paths in a room at any given time.
Recently, conferencing systems have been proposed with multiple audio channels. There is a need for new methods and apparatuses for echo cancelation involving the production and sensing for multiple audio channels.